1. Field of the Invention
This invention generally relates to the fabrication of integrated circuit (IC) electroluminescence (EL) and photoluminescence (PL) devices, and more particularly, to a luminescence device made from a silicon (Si) nanoparticle embedded SiOXNY film, with a high PL quantum efficiency (QE).
2. Description of the Related Art
The fabrication of integrated optical devices involves the deposition of materials with suitable optical characteristics such as absorption, transmission, and spectral response. Thin-film fabrication techniques can produce diverse optical thin films, which are suitable for the production of large area devices at high throughput and yield. Some optical parameters of importance include refractive index (n) and the optical band-gap, which dictate the transmission and reflection characteristics of the thin film.
Typically, bilayer or multilayer stack thin-films are required for the fabrication of optical devices with the desired optical effect. Various combinations of the metal, dielectric, and/or semiconductor layers are also used to form multilayer films with the desired optical characteristics. The selection of the material depends on the target reflection, transmission, and absorption characteristics. While a single layer device would obviously be more desirable, no single thin-film material has been able to provide the wide range of optical dispersion characteristics required to get the desired optical absorption, band-gap, refractive index, reflection, or transmission over a wide optical range extending from ultraviolet (UV) to far infrared (IR) frequencies.
Silicon is the material of choice for the fabrication of optoelectronic devices because of well-developed processing technology. However, the indirect band-gap makes it an inefficient material for optoelectronic devices. Over the years, various R&D efforts have focused on tailoring the optical function of Si to realize Si-based optoelectronics. The achievement of efficient room temperature light emission from the crystalline silicon is a major step towards the achievement of fully Si-based optoelectronics.
The fabrication of stable and reliable optoelectronic devices requires Si nanocrystals with high photoluminescence (PL) and electroluminescence (EL) quantum efficiency. EL is both an optical phenomenon and an electrical phenomenon in which a material emits light in response to an electric current passed through it, or in response to a strong electric field. EL can be distinguished from light emission resulting from heat (incandescence) from the action of chemicals (chemoluminescence), the action of sound (sonoluminescence), or other mechanical action (mechanoluminescence). PL is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, PL can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds.
One approach that is being actively pursued for integrated optoelectronic devices is the fabrication of SiOx (x≦2) thin films with embedded Si nanocrystals. The luminescence due to recombination of the electron-hole pairs confined in Si nanocrystals depends strongly on the nanocrystal size. The electrical and optical properties of the nanocrystalline Si embedded SiOxNy thin films depend on the size, concentration, and distribution of the Si nanocrystals. Various thin-film deposition techniques such as sputtering and plasma-enhanced chemical vapor deposition (PECVD), employing a capacitively-coupled plasma source, are being investigated for the fabrication of stable and reliable nanocrystalline Si thin films, which are also referred to herein as nanocrystalline Si embedded insulating thin films.
Conventional PECVD and sputtering techniques have the limitations of low plasma density, inefficient power coupling to the plasma, low ion/neutral ratio, and uncontrolled bulk, and interface damage due to high ion bombardment energy. Therefore, the oxide films formed from a conventional capacitively-coupled plasma (CCP) generated plasma may create reliability issues due to the high bombardment energy of the impinging ionic species. It is important to control or minimize any plasma-induced bulk or interface damage. However, it is not possible to efficiently control the ion energy using the radio frequency (RF) power of CCP generated plasma. Any attempt to enhance the reaction kinetics by increasing the applied power results in increased bombardment of the deposited film, creating a poor quality films with a high defect concentration. Additionally, the low plasma density associated with these types of sources (˜1×108-109 cm−3) leads to limited reaction possibilities in the plasma and on the film surface, inefficient generation of active radicals and ions for enhanced process kinetics, inefficient oxidation, and process and system induced impurities, which limits their usefulness in the fabrication of low-temperature electronic devices.
The pulsed laser and ion implantation of Si in dielectric thin films has also been extensively investigated for the creation of Si nano-particles. However, the ion implantation approach is not suitable for the uniform distribution of the nc-Si particles across the film thickness. Additionally, the particle agglomeration in Si ion implanted and pulsed laser deposited dielectric films typically leads to red shift of the PL/EL spectrum.
A deposition process that offers a more extended processing range and enhanced plasma characteristics than conventional plasma-based techniques, such as sputtering or PECVD, is required to generate and control the particle size for PL and electroluminescent (EL) based device development. A process that can enhance plasma density and minimize plasma bombardment will ensure the growth of high quality films without plasma-induced microstructural damage. A process that can offer the possibility of controlling the interface and bulk quality of the films independently will enable the fabrication of high performance and high reliability electronic devices. A plasma process that can efficiently generate the active plasma species, radicals and ions, will enable noble thin film development with controlled process and property control.
For the fabrication of high quality SiOx and SiOxNy thin films, the oxidation of a grown film is also critical to ensure high quality insulating layer across the nanocrystalline Si particles. A process that can generate active oxygen radicals at high concentrations will ensure the effective passivation of the Si nanoparticles (nc-Si) in the surrounding oxide matrix. A plasma process that can minimize plasma-induced damage will enable the formation of a high quality interface that is critical for the fabrication of high quality devices. Low thermal budget efficient oxidation and hydrogenation processes are critical and will be significant for the processing of high quality optoelectronic devices. The higher temperature thermal processes can interfere with the other device layers and they are not suitable in terms of efficiency and thermal budget, due to the lower reactivity of the thermally activated species. Additionally, a plasma process which can provide a more complete solution and capability in terms of growth/deposition of novel film structures, oxidation, hydrogenation, particle size creation and control, and independent control of plasma density and ion energy, and large area processing is desired for the development of high performance optoelectronic devices. Also, it is important to correlate the plasma process with the thin film properties as the various plasma parameters dictate the thin film properties and the desired film quality depends on the target application. Some of the key plasma and thin-film characteristics that depend on the target application are deposition rate, substrate temperature, thermal budget, density, microstructure, interface quality, impurities, plasma-induced damage, state of the plasma generated active species (radicals/ions), plasma potential, process and system scaling, and electrical quality and reliability. A correlation among these parameters is critical to evaluate the film quality as the process map will dictate the film quality for the target application. It may not be possible to learn or develop thin-films by just extending the processes developed in low density plasma or other high-density plasma systems, as the plasma energy, composition (radical to ions), system pressure, plasma potential, electron temperature, and thermal conditions correlate differently depending on the process map.
Si nanocrystals with sizes in the range of 1-10 nm have shown enhanced optical and electrical properties due to quantum confinement effects. One challenge in the development of high performance nc-Si embedded thin film based EL devices are the creation and control of the nc-Si particles sizes and distribution, the properties of the inter-particle medium, and the nc-Si particle/dielectric interface quality. EL device efficiency strongly depends on the intrinsic light generation efficiency of the thin film medium, light extraction efficiency, electrical conductivity, and the breakdown field strength of the film. An efficient charge injection at low applied voltages is a factor in fabricating practical EL devices. Generally, it is possible to get higher EL power from nano-particle embedded films by increasing the film thickness. However, the applied voltage to achieve the target EL power also increases. If the film thickness is reduced to attain the same field at lower voltages, then the EL power level decreases due to the lower number of nano-particles available for light generation.
Low temperatures are generally desirable in liquid crystal display (LCD) manufacture, where large-scale devices are formed on transparent glass or plastic substrate. These transparent substrates can be damaged when exposed to temperatures exceeding 650 degrees C. To address this temperature issue, low-temperature Si oxidation processes have been developed. These processes use a high-density plasma source such as an inductively coupled plasma (ICP) source, and are able to form Si oxide with a quality comparable to 1200 degree C. thermal oxidation methods.
It would be advantageous if a nc-Si particle dielectric film could be fabricated to exhibit a larger PL/EL response at shorter wavelengths, such as in the range from 475-700 nm. Conventional PECVD and sputtering techniques have the limitations of low plasma density, inefficient power coupling to the plasma, low ion/neutral ratio, and uncontrolled bulk and interface damage due to high ion bombardment energy. The ion implantation approach is not suitable for the uniform distribution of the nc-Si particles across the film thickness. Additionally, the particle agglomeration in Si ion implanted and pulsed laser deposited dielectric films typically lead to red shift of the PL/EL spectrum.